Process of producing tungsten filaments.



. gas is highly important for the purpose of F SS PROCESS OF PRODUCINGTUNGS'IEN FILAMENTS.

No Drawing.

To all whom it may concern:

Be it known that I, ERNST AUGUST KRiirGER, a citizen of the GermanEmpire, residing at Seehausen, Altmark, in Germany, have inventedcertain new and useful Improvements in the Process of Producing "stenwith a small proportion of carbon and boracic acid or other compound ofboron.

My invention consists in a particular manner of carrying out the processof manufacture with the raw material indicated.

According to the invention I first produce a rod consisting of powderedtungsten with a small proportion of carbon and a boron compound,preferably boracic acid, less than 1% of the latter being usuallysuflicient and only a small trace of carbon, about 0.1%. I may use theboracic acid in the form of powder. I moisten the mixture with water. Imay also mix the tungsten powder and carbon alone, and form a rod ofthis mixture,by pressure, and then impregnate the rod in vacuo with aconcentrated solution of boracic acid. The rod is then heated to about1000 or 1200 C. whereby the boracic acid is caused to slowly swell andexpel the air or other gas present from the minute interstices betweenthe tungsten particles. This expulsion of obtaining nonporous wire. Themelting boracic acid cements the metal particles to each other, and atthe same time the superficial oxid layers dissolved by the boracic acid.Thus purely metallic tungsten particles come into intimate contact witheach other, and can by subsequent treatment be sweated or fused togetherto produce a perfectly homogeneous structure. When the temperaturereaches its higher values the boracic acid is probably reduced to boron,or causes carbid of boron to be formed, with the result that an alloy oftungsten and boron or boron carbid is obtained. This is accompanied byconsiderable shrinkage of the mass. After the rod has been kept for sometime at the temperature mentioned, I heat it to a temperature between1200 and 2000 C. by means of electric current, in a stream of hydrogen,whereby the, metal par- Specification of Letters Patent.

Patented Sept. 28,1915.

Application filed August 2'5, 1913. Serial No. 786,488.

ticles are fused together, and a very homogeneous structure, free fromcavities or crevices, is obtained. The rod is then formed into wire bysuitable mechanical treatment adapted for this purpose. The presence ofthe boron reduces the liability to attack by oxygen, so that there is nonecessity for careful exclusion of atmospheric air in the course of themechanical treatment. The rod being free from cavities, the wireobtained is free from those crevices which always occur in forming wireof pure tungsten.

The wire or filament obtained can be used in the manufacture of electricglow lamps.

Inasmuch as the filament of tungsten alloy is quite as efficient as puretungsten filament, for glow lamp purposes, it is in general notnecessary to eliminate the boron by special treatment. As-a matter offact, the heating of the filament in the lamp to a large extenteliminates the boron, which does not blacken the bulb, but forms a thintransparent deposit. The expulsion of the boron from the filament in thebulb also improves the vacuum, the residual air being mechanicallydeposited, and perhaps in part chemically combined.

If it is desired to have a pure tungsten filament, I heat the filamentto incandescence for a lengthy period, in vacuo, say to a temperature of2000 or 2200 C. and then again hammer or roll and draw the filament,with heat. Y

The process described has very considerable advantages over the processof producing filament from pure tungsten. Particles of metallic tungstenare always coated with oxid, which renders it very difiicult to bakethem together, and the result is that minute cavities are left, intowhich air penetrates and causes further oxidation. Ad-

mixture to the tungsten powder, such as silboron and a binding agentcontaining car-.-

bon, the mixture being heated to carbonize the binding agent, and thenheated to white heat by current, in a stream of hydrogen. In thisprocess only so much of the metal is used as will render the mixtureconductive at ordinary temperatures, so that the whole of the tungstenor molybdenum becomes alloyed with boron.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be performed, I declare thatwhat I claim is:

The process of producing a filament of tun sten or tungsten alloy, whichconsists in ormin'g, by pressure, a rod consisting of a mixture ofpowdered tungsten with a small proportion of carbon and boracic acid orother boron compound, heating said rod to a temperature of aboutlOOO to1200 C.

at which the particles cake together, then heating the rod toincandescence by electric current in a stream of hydrogen, and thenreducing the rod to wire by mechanical treatment. s a

In witness whereof I have signed this specification in the presence oftwo witnesses.

ERNST AUGUST KRUGER. Witnesses:

WOLDEMAB HAUPT, HENRY HAsrER.

